What did you notice about the colour of the two stars Betelgeuse and Rigel?
Betelgeuse is red and Rigel is blue in colour.
In Grades 6 and 8 learners covered material regarding the solar system including the Sun. In Grade 7, they focused on the system which includes the Sun, Earth and Moon. Learners should be familiar with the fact that the Sun is a star and produces heat and light (energy) via nuclear reactions. In this chapter the focus is on the life cycle of stars, including how they are born and die. The exact evolution that a star follows depends on the initial mass of the star. The Sun's evolution is presented as an example. The main aims of this chapter are to ensure that learners understand the following:
stars spend most of their lives on the main sequence fusing hydrogen gas to helium gas
Some learners may ask why stars look 'spiky' in the photographs from telescopes, but in the diagrams shown here, they are presented as spheres. Watch this video to find out and explain to your learners:
Do you think it is important to teach astronomy to learners at school? Read this interesting and informative article detailing the benefits and applications of astronomy: [link] http://www.iau.org/public/themes/why_is_astronomy_important/
5.1 The birth of a star (0.5 hours)
5.2 Life of a star (1 hour)
Activity: Observing Orion in the spring sky
5.3 Death of a star (1.5 hours)
Activity: Life cycle of a Sun-like star
Activity: The life cycle of the Sun
Activity: Flow diagram poster showing the lifecycle of a Sun-like star
writing, drawing, sequencing
A good way to introduce the topic of stellar evolution is to start by asking learners how long they think stars last . Many will answer forever. Many people are unaware that, like humans, stars are born, live their lives and then die. You can also ask them what is meant by 'living' when referring to a star, after all, stars do not perform the seven life processes, as taught in Life and Living. Astronomers generally consider stars that are undergoing nuclear reactions in their cores to be living stars.
Stars are also compared in terms of relative concepts, such as:
Stars do not live forever, just like people. Stars are born, live their lives, changing or evolving as they age, and eventually they die. Often stars do this in a much more spectacular way than humans do!
In this chapter, you will notice that many nouns are used as adjectives, for example, Sun is the noun and solar is the adjective. Other noun and adjective pairs include: moon and lunar, star and stellar, planet and planetary.
Scientists speak of stellar evolution when talking about the birth, life and death of stars. The lifetime of individual stars is way too long for humans to observe the evolution of a single star, so how do scientists study stellar evolution? This is possible as there are so many stars in our galaxy, so we can see lots of them at different stages of their lives. In this way, astronomers can build up an overall picture of the process of stellar evolution. In this chapter you will discover how stars are born, how they evolve, and how they die.
Lots of work went into figuring out the processes in stellar evolution. This work is still on going. What you are learning here in Natural Sciences is based on years of research, and current research is constantly taking place, updating what we know.
Stellar evolution explained (full documentary).
In this section learners will discover that stars are born in giant clouds of dust and gas, called nebulae, in space. In order to understand how collapsing gas clouds heat up to eventually form stars, learners need to understand that compressing a gas heats it up and that allowing a gas to expand cools it down. If they are unfamiliar with this concept a good analogy is to think about over-inflating a bicycle tyre (without bursting it). You could demonstrate this in class by getting learners to slightly over-inflate a tyre. They will find that the pump and tyre get hot!
In the case of inflating a tyre, you are forcing more and more molecules into a given volume (assuming that the tyre is now at full capacity). So you are compressing or squeezing the gas. Each molecule has a certain amount of kinetic energy. As more molecules are forced in by the pump, the air in the tyre is compressed and the total thermal energy increases because there are more molecules colliding inside the tyre. As more particles are contained in the same volume, the air's temperature in the tyre increases. As you deflate the tyre, you allow the gas to expand, the molecules are more spread out. There is then less thermal energy and so the temperature decreases. You could let students feel the air as it is released from the tyre - it should be colder than the ambient air as it is rapidly expanding as it escapes from the tyre.
Stars are born in vast, slowly rotating, clouds of cold gas and dust called nebulae (singular nebula). These large clouds are enormous, they have masses somewhere between 100 thousand and two million times the mass of the Sun and their diameters range from 50 to 300 light years across.
A light year is the distance that light travels in one year. Light travels extremely fast at 299 792 458 m/s. One light year is equivalent to 10 trillion kilometers.
How far is a light year?
A famous example of one of these huge clouds is the Orion nebula in the constellation of Orion. It is visible with the naked eye if the sky is dark enough. These clouds are so massive that they can collapse under their own gravity if they are disturbed.
The collapse of a star can be triggered when the cloud is squeezed. For example if a cloud passes through a spiral arm in a galaxy it will be slowed down and compressed. This explains why lots of stars are formed in the spiral arms of galaxies.
Over time the clouds contract, become denser and slowly heat up. The clouds also break up into smaller clumps. As the clumps get smaller they begin to flatten out into a disk shape. The centre of each clump will eventually contain a star and the outer disk of gas and dust may eventually form planets around the star.
Star birth (Orion Nebula).
As the contracting clump continues to heat up, a protostar is formed at the centre. A protostar is a dense ball of gas that is not yet hot enough at the centre to start nuclear reactions. This stage lasts for roughly 50 million years. As the collapse continues, the mass of the protostar increases, squeezing it further and increasing the temperature. If the protostar is massive enough for the temperature to reach 10 million degrees Celsius, then it becomes hot enough for nuclear reactions to start and the protostar will will technically be referred to as a star.
The Coronet cluster, shown in the image, has a host of young stars at different life stages, which allows astronomers to gather data and pinpoint details of how the youngest stars evolve.
Do you remember that we learned about nuclear reactions last term in Energy and Change when looking at nuclear power plants?
The young star starts converting hydrogen to helium via nuclear fusion reactions. Nuclear reactions in stars produce vast amounts of energy in the form of heat and light, which is radiated into space. This energy production prevents the star from contracting further. As the star shines, the disk of dust and gas surrounding the star is slowly blown away by the star's stellar wind which leaves behind any planets if they have already formed.
The end stages of star formation (birth).
Just like the Sun loses particles into space in the form of the solar wind, other stars also have winds called stellar winds.
In the upper left of the image of Large Magellanic Cloud, you can see a collection of blue and white young stars. They are extremely hot and are some of the most massive stars known anywhere in the Universe.
The image shown here of the Large Magellanic Cloud, a satellite galaxy to the Milky Way, illustrates very clearly, an example of sequential star formation, where new star birth is triggered by the previous generation of massive stars. You can point some of these observations out to learners:
Learners may ask why some of these images have black boxes in the top, right corner, as though some of the image is missing. These strange, stair-shaped images come from the Hubble Telescope's Wide Field and Planetary Camera 2, or WFPC2. WFPC2 consists of four cameras, each of which takes a picture of a section of the target. It is like taking four pictures of a single scene, then putting them together to create the whole picture. But one of WFPC2's cameras, the top right, takes a magnified view of the section it is observing, to allow astronomers to study that section in finer detail. When the images are processed, that magnified section is shrunk down to the same size as the other sections, so that it fits into the image, resulting in the stair-shaped pattern. You can read more about it here: http://hubblesite.org/gallery/behind_the_pictures/wacky_shape/constructing.php
Curious about the Universe, but don't know where to start? Have a look at this step-by-step guide to becoming an awesome amateur astronomer. http://www.unawe.org/awesome/
This section covers the main stages of a star's life, from infancy to old age. Learners will also discover why stars do not all look the same and why they evolve at different rates and have different lifetimes: it is a consequence of having different masses. They will learn how important the mass of a star is in determining its evolution and observable characteristics.
Stars: Life and death.
A star is considered to be 'born' once nuclear fusion reactions begin at its centre. Initially hydrogen is converted to helium deep inside the star. A star that is converting hydrogen to helium is called a main sequence star. Stars spend most of their lives as main sequence stars, converting hydrogen to helium at their centres or cores. A star may remain as a main sequence star for millions or billions of years.
Most of the stars in the Universe, about 90 %, are main sequence stars. The Sun is a main sequence star.
Main sequence stars are not all the same. They have different masses when they are born, depending on how much matter is available in the nebula from which they formed. These stars can range from about a tenth of the mass of the Sun up to 200 times as massive. Different mass stars have different observable properties.
We normally associate red with being hot and blue with being cold. But, in stars, the bluer the star, the hotter it is, and the redder it is, the older and colder it is.
Main sequence stars also have different colours, depending on the temperatures of their surfaces. Look at the following picture and correctly label the temperatures of all the stars using the list of temperatures below. Which star represents our Sun?
Temperature list: 3000 °C, 4500 °C, 6000 °C, 10 000 °C, 40 000 °C
The following image shows the correct labels for the temperatures of different stars:
The yellow star represents our Sun.
Why are hotter stars bluer in colour? Can you remember what you learnt about the spectrum of visible light in Grade 8? The colour blue corresponds to light at shorter wavelengths (higher frequencies) than the colour red. Shorter wavelengths (higher frequencies) correspond to higher energies and thus hotter temperatures. This is also seen in the flames of a fire or candle. If you look at the flames, the central regions are bluer (and hotter) than the outer regions, which are orange and yellow.
The biggest stars in the Universe.
The colours of stars.
Orion is an easily recognisable constellation visible in cities as well as in dark skies. In this activity learners will have to look at the night sky to spot the constellation and identify the stars Betelgeuse and Rigel and note their difference in colour. Orion is up in the east from around 00:30 at the beginning of October, however as the months progress it rises earlier. By the beginning on December Orion is visible from around 20:30 in the east. If observing the constellation is unfeasible, you could ask learners to look at the image of the constellation in this chapter instead.
Betelgeuse is so huge that, if it replaced the Sun at the center of our solar system, its outer atmosphere would extend past the orbit of Jupiter (see the scale at lower left of the image).
At the beginning of October Orion is visible in the east from around 00:30 until morning. From the beginning of November Orion is visible in the east from around 22:30 and from the beginning of December it is visible in the east from around 20:30.
What did you notice about the colour of the two stars Betelgeuse and Rigel?
Betelgeuse is red and Rigel is blue in colour.
Why do you think the stars look different? Hint: Look back at the colours of stars in the diagram before this activity to see what this tells us about their temperatures.
Rigel is much hotter than Betelgeuse, hence it is bluer.
How long a main sequence star lives depends on how massive it is. More massive stars move onto the next stages of their lives more quickly than lower mass stars. In fact they are main sequence stars for a shorter time than lower mass stars.
A higher-mass star might have more material, but it also uses up the material more quickly due to its higher temperature. For example, the Sun will spend about 10 billion years as a main sequence star, but a star 10 times as massive will last for only 20 million years. A red dwarf, which is half the mass of the Sun, can last 80 to 100 billion years.
When the hydrogen in the centre of the star is depleted, the star's core shrinks and heats up. This causes the outer part of the star, the star's atmosphere, which is still mostly hydrogen, to start to expand. The star becomes larger and brighter and its surface temperature cools so it glows red. The star is now a red giant star. Betelgeuse, as you observed in the last activity, is a red giant star.
Globular clusters are particularly useful for studying stellar evolution, since all of the stars in the cluster have the same age (about 10-15 billion years), but cover a range of stellar masses.
Why does a red giant glow red?
It is red because it has cooled compared to when it was a main sequence star.
Why do you think red giants are called "giant" stars?
It is called a giant because the outer layers have expanded outwards and the star has got much larger than it was when it was a main sequence star.
Eventually the core of the star becomes hot enough for the next nuclear reaction to start: atoms of helium collide and fuse into heavier elements such as carbon and oxygen. However, eventually the helium in the core will also be depleted. From this point onwards, the fate of the star is determined by its mass.
Scientists discover star devouring nearby planet.
For medium-sized stars, such as the Sun, the temperature in their centres will never get high enough to fuse the newly-formed carbon and oxygen into heavier elements and so they do not evolve much further. Following the red giant phase, the star becomes unstable and will eventually die as you will discover in the next section.
Scroll through this interactive animation to get a sense of the scale of some of the stars and other objects in our Universe. http://htwins.net/scale2/
The animation listed in the Visit box provides a very useful tool to give learners a sense of the scale of the Universe. If possible, you can project it up in your classroom and scale through it from a human all the way out until you get to some of the massive supergiants, and then beyond. You will also be able to see the scale of some of the objects mentioned in this chapter, such as the Crab Nebula, the Large Magellanic Cloud and Pillars of Creation.
In this section learners will discover how stars die. The focus is on the death of a low mass star like the Sun. However, for completeness, the way that high mass stars die is also briefly mentioned. There are two activities in this section related to the life of Sun-like stars. Both of these are intended to help learners remember and understand the sequence of phases that a star like the Sun undergoes during its life. There is a lot of unfamiliar terminology in stellar evolution and it can be confusing for learners. Hopefully by doing activities rather than simply reading about the different stages in a Sun-like star's evolution, learners will find the subject easier to understand.
As a star enters the final stages of its life, after it has become a red giant, the star becomes unstable and expands and contracts over and over. This causes the star's outer layers to become detached from the central part of the star and they gently puff off into space. When the last of the gas in the star's outer layers is blown away, it forms an expanding shell around the core of the star called a planetary nebula. Planetary nebulae glow beautifully as they absorb the energy emitted from the hot central star. They can be found in many different shapes, as shown in the following images.
The plural of nebula is nebulae. Planetary nebulae have nothing to do with planets but were named like this in the 1700s because they resembled planets when observed with the telescopes of the time.
A planetary nebula is different to a stellar nebula. A stellar nebula is where stars are born, whereas a planetary nebula is what some stars form at the end of their lives.
The Butterfly Nebula is a dying star that was once five times the mass of the Sun. What resembles the butterfly wings are actually hot clouds of gas tearing across space at almost 1 million km an hour - fast enough to travel from Earth to the Moon in 24 minutes!
Earth consumed by red giant star in 5 billions years time.
A tour of a planetary nebula.
Some time after puffing off its outer layers, the central star will run out of fuel. When this happens the central star begins to die. Gravity causes the star to collapse inwards and the star becomes incredibly dense and compact, about the size of the Earth. The star has then become a white dwarf star.
White dwarfs have this name because of their small size and because they are so hot that they shine with a white hot light. The central parts of stars are much hotter than their surfaces, and a white dwarf is made from the remaining central parts of a star which explains why they are so hot.
The following image shows the relative size of Sirius B, a nearby white dwarf star, compared to some of the planets in our solar system. Stars and stellar remains can be smaller than planets.
White dwarfs no longer produce energy via nuclear reactions and so as they radiate their energy into space in the form of light and heat. They slowly cool down over time. Eventually, once all of their energy is gone, they no longer emit any light. The star is now a dead black dwarf star and will remain like this forever.
White dwarf stars are so dense that one teaspoon of material from a white dwarf would weigh up to 100 000 kg.
This activity can be performed in pairs or small groups. This activity demonstrates the life of a Sun-like star using a yellow balloon to represent the Sun. Learners must follow the instructions to demonstrate each of the phases that a star like the Sun goes through during its life. This activity is best completed in pairs where one member "gives the orders" and the other member completes the activity. If you have time you can repeat the activity, swapping the pairs around.
1) Star is born
Blow up the balloon to about 6 cm in diameter
2) 5 million years
3) 10 million years
4) 500 million years
Wait - planets are being formed around the star.
5) 1 billion years
Blow the balloon up a little bit
6) 9 billion years
Blow up the balloon some more and colour it
red - it is now a red giant star
7) 10 billion years
Blow the balloon up a little bit. The outer layers are now being blown off. To simulate this, slowly allow the balloon to deflate. Cut the balloon into pieces and scatter them around the white ball. The star has now become a white dwarf (the ball) surrounded by a planetary nebula (the pieces of balloon).
8) 50 billion years
Move the planetary nebula farther away from the white dwarf.
9) 500 billion years
Remove the planetary nebula and colour the ball
black - the star is now a black dwarf.
The different stages of evolution of a star like the Sun are summarised in the diagram below and compared to the lifecycle of a person.
Let's take a closer look at the life of our star, the Sun.
How the Sun will die.
The Sun is currently about halfway through its lifetime as a __________ star. In about 4.5 billion years time the Sun will swell up to form a __________ star engulfing the Earth as it does so.
The Sun is currently about halfway through its lifetime as a main sequence star. In about 4.5 billion years time the Sun will swell up to form a red giant star engulfing the Earth as it does so.
After the Sun has become a red giant, it will eventually become unstable and puff off its outer layers forming a beautiful __________ . The central core of the Sun will be left exposed in the centre of the planetary nebula.
After the Sun has become a red giant, it will eventually become unstable and puff off its outer layers forming a beautiful planetarynebula. The central core of the Sun will be left exposed in the centre of the planetary nebula.
Once the fuel runs out in the core of the Sun, nuclear reactions will __________. The Sun will then have become a hot __________ star, left behind at the centre of the planetary nebula.
Once the fuel runs out in the core of the Sun, nuclear reactions will stop. The Sun will then have become a hot white dwarf star, left behind at the centre of the planetary nebula.
As there are no ongoing nuclear reactions, as the white dwarf shines it slowly cools and will eventually form a __________ dwarf.
As there are no ongoing nuclear reactions, as the white dwarf shines it slowly cools and will eventually form a black dwarf.
In this activity learners will make a poster showing the different stages of stellar evolution experienced by a Sun-like star. The idea is to create a flow diagram showing which stage leads on to the next. Learners can use photographs or pictures printed from the internet or they may draw their own pictures depending on time and resources available. An example is presented below for guidance.
If learners have access to the internet, they can print out images of the various stages. otherwise they can use the reference diagrams in the workbook to draw pictures.
Advanced: Write down approximately how long each stage lasts. You can use the timeline of the Sun's evolution in this chapter to help you.
Where are stars born?
In vast cold clouds of gas and dust called nebulae.
Why is a red giant so named?
It is called a red giant because it is red in colour and much larger than a main sequence star.
What kind of stellar remnant is left behind once a star like the Sun dies?
A white dwarf star.
What is a planetary nebula?
A glowing nebula formed by an expanding shell of gas around an aging star.
How big is a white dwarf?
About the size of the Earth.
The following content on supernovae is not in CAPS, but has been included here as the stellar evolution discussed previously explains small and medium-sized stars. Giant stars have a different end, as discussed here.
So far we have looked at stars that are about the same mass as our Sun. But, what about stars that are more massive? How do they die?
Here we are not talking aboutbigger stars, but rather stars that are more massive. It is not the size that counts, but the mass of the star.
Stars more than eight times the mass of the Sun end their lives spectacularly. When the hydrogen at their cores becomes depleted, they swell into red supergiants which are even larger than red giants.
The temperature in the cores of these supergiants gets high enough for them to fuse elements heavier than hydrogen and helium.
A red supergiant can fuse successively heavier and heavier elements for a few million years until its core is filled with iron. At this point, nuclear reactions stop and the star collapses rapidly under its own gravity. The collapsing outer layers of the star hit the small central core with such a force that they rebound and send a ripple outwards through the star blowing the outer layers of the star into space in a huge explosion called a supernova.
The elements made inside stars are scattered through space when the outer layers of the stars are blown off either in planetary nebulae or supernovae. This stardust is recycled and used in forming the next generation of stars and planets. The calcium in our bones and the iron in our blood were all made inside stars.
The Crab Nebula.
Japanese and Chinese astronomers recorded the violent supernova event that led to the Crab Nebula nearly 1,000 years ago in 1054.
For a week or so, a supernova can outshine all of the other stars in its galaxy. However, they quickly fade over time. The central star left behind is either made of neutrons and it is called a neutron star, or if the initial star was really massive, a black hole forms. The leftover neutron star or black hole is surrounded by an expanding cloud of very hot gas.
A black hole is a region of space where gravity is so strong that even light cannot escape. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star dies. As light cannot escape you cannot actually see a black hole. Black holes can be detected by their gravitational effects on nearby visible objects, or in the case of a black hole that is actively absorbing material from its surroundings, the material may emit light before it is sucked into the black hole. As well as stellar mass black holes, there are much more massive black holes in the centres of galaxies, called supermassive black holes.
The largest black holes in the Universe and what's inside black holes
In February 1987, astronomers observed a supernova explosion, called Supernova 1987A. It is one of the brightest stellar explosions observed since the invention of the telescope 400 years ago. The supernova belongs to the Large Magellanic Cloud, a nearby galaxy about 168 000 light-years away. Even though the stellar explosion took place around 166 000 BC, its light arrived here less than 25 years ago.
Supernovae have also been observed previously with the naked eye before the invention of the telescope. On 9 October 1604, sky watchers, including astronomer Johannes Kepler, spotted a "new star" in the sky. Now, we have images of the remnants of the supernova and know that it is not a new star, but rather the death of a massive star.
Kepler's supernova was the last exploding supernova seen in our Milky Way galaxy.
How many people are in space right now? Find out here. http://www.howmanypeopleareinspacerightnow.com/
The concept map on the life cycle of stars has been started, but you need to finish it by summarising the concepts for each stage, namely birth, life and death of a star.
What is the name of the giant clouds where stars are formed? [1 mark]
Nebulae (singular nebula).
In the human life cycle, a foetus is the unborn baby in a mother's womb. What is the equivalent stage in a star's life called? [1 mark]
It is called a protostar.
Under what conditions do astronomers technically say a star has been born? [1 mark]
If there is enough gas and dust for the temperature to become hot enough for nuclear reactions to start, the protostar will then technically be called a star.
Which star colour is hotter, white or yellow? [1 mark]
A white star is hotter than a yellow star.
What nuclear reaction does a main sequence star undergo? [2 marks]
A main sequence star burns hydrogen to helium at its core. This is called nuclear fusion.
Once the Sun has exhausted its hydrogen fuel supply, it will swell up to form what type of star? [1 mark]
A red giant.
Low mass stars like the Sun eject their outer layers. What is the name of the object they form when they do this? [1 mark]
Low mass stars eject their outer layers forming a planetary nebula.
What kind of star is left behind after a planetary nebula? [1 mark]
A white dwarf.
What is the difference between a stellar nebula and a planetary nebula? [2 marks]
A stellar nebula is where stars are born, whereas a planetary nebula is what a star forms at the end of its life.
Study the following diagram showing a star's evolution.
Provide labels for the different stages. [5 marks]
What changes occur from stage B to form C? [2 marks]
Some time after puffing off its outer layers at stage D, the fuel of the central star will have become depleted. What causes the star to collapse inwards to become E? [1 mark]
What eventually happens to the star after stage E? [1 mark]
Main sequence star/Yellow star
When the hydrogen in the centre of the star is depleted, the star's core shrinks and heats up. The outer part of the star, which is still mostly hydrogen, starts to expand. The star becomes larger, brighter and its surface temperature cools so it glows red. The star is now a red giant star.
Gravity causes the star to collapse inwards and form a very dense star.
The energy of the white dwarf will have become depleted and it stops emitting light and becomes a black dwarf star forever more.
Massive stars die in powerful explosions. What are these explosions called? [1 mark]
Supernovae explosions (singular supernova).
Total [21 marks]